The production limit for a particular fuel in petroleum refineries is controlled by factors such as crude capacity, individual process capacities, availability of particular crudes and total petroleum product demand. These factors interact in a complex physical and economic way such that at a particular point in time a refiner may be unable to supply the demand for a particular premium fuel, such as jet fuel meeting Jet-A specifications, from a straight-run kerosene fraction. The term "premium fuels" as used herein refers broadly to distillate fuels boiling within the range of 250.degree. F. to 700.degree. F., and more specifically to jet fuel, kerosene, naphtha, diesel fuel and heating oils.
Conventional catalytic hydrodesulfurization (CHD) is a well known process for reducing the sulfur content of a virgin kerosene of otherwise suitable quality to bring such a feed into conformance with the sulfur specification for jet fuel. Typical CHD catalysts contain from about 2 to about 4 wt % cobalt and about 8 to about 10.5 wt % molybdenum on an alumina support. The CHD process with a kerosene feed typically is operated at 450.degree. F. to 750.degree. F., at a hydrogen partial pressure of 200 to 800 psia, at a liquid hourly space velocity of 0.5 to 5.0, and with 400 to 3000 standard cubic feet per barrel (SCF/Bbl) of hydrogen circulation to effect 60% to 90% desulfurization. There are a number of commercially available catalyts which differ in the nature of the support, amount of metal, etc., and also a number of known process variations. The CHD process per se is not considered a part of this invention. The described use of the commonplace CHD process to upgrade a high sulfur virgin kerosene feedstock to jet fuel specifications is representative of the use of a "conventional" feedstock for a premium distillate fuel.
Conventional hydrocracker (HDC) recycle has a sulfur content well within the specification for jet quality fuel, but an unacceptably high freeze point. Although very small amounts of HDC recycle might be blended with CHD treated virgin kerosene to augment the supply of jet fuel, this would furnish only a very small incremental supply of this premium fuel.
Processes for dewaxing petroleum distillates have been known for a long time. Dewaxing, as is well known, is required when highly paraffinic oils are to be used in products which need to remain mobile at low temperatures. The higher molecular weight straight chain normal and slightly branched paraffins which are present in oils of this kind are waxes which are the cause of high pour points in the oils and if adequately low pour points are to be obtained, these waxes must be wholly or partly removed. Catalytic dewaxing processes achieve this objective by selectively cracking the longer chain n-paraffins to produce lower molecular weight products which may be removed by distillation. Processes of this kind are described, for example, in The Oil and Gas Journal, Jan. 6, 1975, pages 69 to 73 and U.S. Pat. No. 3,668,113, incorporated herein by reference for background purposes.
In order to obtain the desired selectivity in catalytic dewaxing, the catalyst has usually comprised an acidic zeolite having a pore size which admits only the straight chain n-paraffins, or these along with only slightly branched chain paraffins, but which excludes more highly branched materials, cycloaliphatics and aromatics. Zeolites such as ZSM-5, ZSM-11, ZSM-12, ZSM-23, ZSM-35 and ZSM-38 have been proposed for this purpose in dewaxing processes and their use is described in U.S. Pat. Nos. 3,894,938; 4,176,050; 4,181,598; 4,222,855; 4,229,282 and 4,247,388. A dewaxing process employing synthetic offretite is described in U.S. Pat. No. 4,259,174. The content of these patents are incorporated by reference for background purposes.
Since the dewaxing processes just described function by means of cracking reactions, a number of useful products become degraded to lower molecular weight materials. For example, distillate and naphtha range materials may be cracked to butane, propane, ethane and methane, and so may the lighter n-paraffins which do not contribute to the waxy nature of the oil. These lighter products are generally of lower value than higher molecular weight materials.
U.S. 4,419,220 to La Pierre et al. discloses that distillate feedstocks may be effectively dewaxed by isomerizing the waxy paraffins without substantial cracking. The isomerization is effected with a Zeolite Beta catalyst and may be conducted either in the presence or absence of added hydrogen. The catalyst should include a hydrogenation component such as platinum or palladium in order to promote the reactions which occur. The hydrogenation component may be used to promote certain hydrogenation/dehydrogentation reactions which will take place during the isomerization. The process described in U.S. Pat. No. 4,419,220 is carried out at elevated temperature and pressure. Temperatures will normally be from 250.degree. C. to 500.degree. C. (about 480.degree. F. to 930.degree. F.) and pressures from atmospheric up to 25,000 kPa (3,600 psig). Space velocities will normally be from 0.1 to 20. The entire content of U.S. Pat. No. 4,419,220 is herein incorporated by reference as if fully set forth.
The foregoing hydrodewaxing process may be used to dewax a variety of feedstocks ranging from relatively light distillate fractions up to high boiling stocks, including waxy distillate stocks such as gas oils, kerosenes, jet fuels, lubricating oil stocks, heating oils and other distillate fractions whose pour point and viscosity need to be maintained within certain specification limits. Hydrocracked stocks are a convenient source of stocks of this kind and also of other distillate fractions since they normally contain significant amounts of waxy n-paraffins which have been concentrated by the hydrocracking of naphthenes and aromatics. The feedstock will normally be about a C.sub.10 + feedstock containing paraffins, olefins, naphthenes, aromatics and heterocyclic compounds and with a substantial proportion of higher molecular weight n-paraffins and slightly branched paraffins which contribute to the waxy nature of the feedstock. During the processing, the n-paraffins become isomerized to iso-paraffins and the slightly branched paraffins undergo isomerization to more highly branched aliphatics. At the same time, a measure of cracking does take place so that not only is the pour point reduced by reason of the isomerization of nparaffins to the less waxy branched chain iso-paraffins but, in addition, the heavy ends undergo some cracking or hydrocracking to form liquid range materials which contribute to a low viscosity product. The degree of cracking which occurs is, however, limited so that the gas yield is reduced, thereby preserving the economic value of the feedstock.
It is an object of this invention to provide a hydrodesulfurization-hydrodewaxing process for the manufacture of premium distillate fuels from two different feeds, one of which requires hydrodesulfurization and the other of which requires dewaxing. It is a further object of this invention to provide such a process utilizing a single reactor containing a hydrodesulfurization catalyst and a hydrodewaxing catalyst. It is a still further object to provide a process for manufacturing jet quality fuel from kerosene and hydrocracker recycle by passing each feed in blocked oleration and hydrogen gas over an HDS catalyst and- a dewaxing catalyst, in that order, and blending the effluents from the two blocked feeds. These and further objects will become evident from reading this entire specification including the appended claims.